You summon a swarm of spiders, which attacks all other creatures within its area. (You may summon the swarm so that it shares the area of other creatures.) If no living creatures are within its area, the swarm attacks or pursues the nearest creature as best it can. The caster has no control over its target or direction of travel. You may instead summon 1d2 x creatures from the Summon Spider I spell. These cannot be summoned so that they share the area of another creature, but they will each attack a random creature adjacent to them (if given a choice between the caster or one or more of her alies and another creature, they will attack the other creature). Otherwise, they will act in the same way as the swarm would.